Residual moisture reduction in coarse coal centrifuges. 4. Theory, process engineering and costs

Air purging is a new way of reducing the moisture content of coarse coal (i.e. within the 50–0.5 mm size range) products from vibrating basket centrifuges. The process, which involves injecting a turbulent stream of high velocity air through the coal bed as it traverses the centrifuge basket, has recently been shown to work at commercial scale. This paper describes some background modelling and experimental work that was undertaken to enhance understanding of the technique. st critical variables affecting air purging are the speed of the air exiting the bed and the height of the bed itself. Both parameters were characterised at commercial scale through a combination of experimental tests and modelling. These data were then used to predict the flow rate of the air into the bed and also the pressure drop across it. The predicted values agreed well with the experimental data. Based on this information it was then possible to establish that the operating costs of air purging would be of the order of A$0.02–0.03/ton of coal, considerably less than the financial gains associated with drier products. The next stage of assessing air purging would involve prolonged trials in different plant situations to resolve the remaining engineering issues. The technology is now sufficiently well understood for these trials to be approached with some confidence.